Impera've Programming

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Jane Sullivan
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1 Impera've Programming Python Programs Interac)ve Input/Output One- Way and Two- Way if Statements for Loops User- Defined Func)ons Assignments Revisited and Parameter Passing

using an IDE or from the command line line2 = 'Welcome to the world of Python!

2 Python program line1 = 'Hello Python developer...' A Python program is a sequence of Python statements Stored in a text file called a Python module Executed using an IDE or from the command line line2 = 'Welcome to the world of Python!' print(line1) print(line2) line1 = 'Hello Python developer...' line2 = 'Welcome to the world of Python!' print(line1) print(line2) hello.py $ python hello.py Hello Python developer Welcome to the world of Python!

Built- in func'on print() Func)on print() prints its input argument to the IDLE window The argument can be any object: an integer, a float, a string, a list, o Strings are printed without quotes

3 Built- in func'on print() Func)on print() prints its input argument to the IDLE window The argument can be any object: an integer, a float, a string, a list, o Strings are printed without quotes and to be read by people, rather than to be interpreted by Python, The string representa)on of the object is printed print(0) 0 print(0.0) 0.0 print('zero') zero print([0, 1, 'two']) [0, 1, 'two']

Built- in func'on input() Func)on input() requests and reads input from the user interac)vely It s (op)onal) input argument is the request message Typically used on the right side of an assignment

The string typed by the user is assigned to the variable on the ley side of the assignment statement name = input('enter your name: ') Enter your name: Michael name 'Michael' ========= RESTART

4 Built- in func'on input() Func)on input() requests and reads input from the user interac)vely It s (op)onal) input argument is the request message Typically used on the right side of an assignment statement When executed: 1. The input request message is printed 2. The user enters the input 3. The string typed by the user is assigned to the variable on the ley side of the assignment statement name = input('enter your name: ') Enter your name: Michael name 'Michael' ========= RESTART ============= Enter your first name: Michael Enter your last name: Jordan Hello Michael Jordan... Welcome to the world of Python! first = input('enter your first name: ') last = input('enter your last name: ') line1 = 'Hello + first + '' + last + ' ' print(line1) print('welcome to the world of Python!') input.py

Built- in func'on eval() Func)on input() evaluates anything the user enters as a string What if we want the user to interac)vely enter non- string input such as a number?

5 Built- in func'on eval() Func)on input() evaluates anything the user enters as a string What if we want the user to interac)vely enter non- string input such as a number? Solu)on 1: Use type conversion Solu)on 2: Use func)on eval() o Takes a string as input and evaluates it as a Python expression age = input('enter your age: ') Enter your age: 18 age '18' int(age) 18 eval('18') 18 eval('age') '18' eval('[2,3+5]') [2, 8] eval('x') Traceback (most recent call last): File "<pyshell#14>", line 1, in <module> eval('x') File "<string>", line 1, in <module> NameError: name 'x' is not defined

6 Exercise Write a program that: 1. Requests the user s name 2. Requests the user s age 3. Computes the user s age one year from now and prints the message shown Enter your name: Marie Enter your age: 17 Marie, you will be 18 next year! name = input('enter your name: ') age = int(input('enter your age: ')) line = name + ', you will be ' + str(age+1) + ' next year!' print(line)

7 Exercise Write a program that: 1. Requests the user s name 2. Requests the user s age 3. Prints a message saying whether the user is eligible to vote or not Need a way to execute a Python statement if a condi)on is true

8 One- way if statement if <condition>: <indented code block> <non-indented statement> if temp > 86: print('it is hot!') print('be sure to drink liquids.') print('goodbye.') The value of temp is temp > 86: True print('it is hot!') False print('be sure to drink liquids.') Print('Goddbye.')

Exercises Write corresponding if statements: a) If age is greater than 62 then print 'You can get Social Security benefits b) If

c) If hits is greater than 10 and shield is 0 then print "You're dead.

print('vacation > 10 and can shield get time!') Social == 0: Security benefits') print("you're dead.

9 Exercises Write corresponding if statements: a) If age is greater than 62 then print 'You can get Social Security benefits b) If string 'large bonuses' appears in string report then print 'Vacation time! c) If hits is greater than 10 and shield is 0 then print "You're dead..." age report hits = = 45= 12'no bonuses this year' if shield age 'large > = 62: 0bonuses' in report: if hits print('you print('vacation > 10 and can shield get time!') Social == 0: Security benefits') print("you're dead...") age report = 65= 'large bonuses this year' You're if age 'large dead... > 62: bonuses' in report: hits, print('you print('vacation shield = 12, can 2get time!') Social Security benefits') if hits > 10 and shield == 0: print("you're dead...") You Vacation can get time! Social Security benefits

') print('goodbye.') temp > 86: True temp > 86: True print('it is hot!

10 Indenta'on is cri'cal if temp > 86: print('it is hot!') print('drink liquids.') print('goodbye.') if temp > 86: print('it is hot!') print('drink liquids.') print('goodbye.') temp > 86: True temp > 86: True print('it is hot!') print('it is hot!') False False print('drink liquids.') print('drink liquids.') print('goddbye.') print('goddbye.')

Two- way if statement if <condition>: <indented code block 1> else:

90. if temp > 86: print('it is hot!') print('be sure to drink liquids.

') False temp > 86: True print('it is not hot!') print('it is hot!

11 Two- way if statement if <condition>: <indented code block 1> else: <indented code block 2> <non-indented statement> The value of temp is if temp > 86: print('it is hot!') print('be sure to drink liquids.') else: print('it is not hot.') print('bring a jacket.') print('goodbye.') False temp > 86: True print('it is not hot!') print('it is hot!') print('bring a jacket.') print('be sure to drink liquids.') print('bring a jacket.')

============RESTART================ Enter your name: Marie Enter your age: 18 Marie, you can vote.

12 Exercise Write a program that: 1) Requests the user s name 2) Requests the user s age 3) Prints a message saying whether the user is eligible to vote or not Enter your name: Marie Enter your age: 17 Marie, you can't vote. ============RESTART================ Enter your name: Marie Enter your age: 18 Marie, you can vote. name = input('enter your name: ') age = eval(input('enter your age: ')) if age < 18: print(name + ", you can't vote.") else: print(name + ", you can vote.")

13 Execu'on control structures The one- way and two- way if statements are examples of execu)on control structures Execu)on control structures are programming language statements that control which statements are executed, i.e., the execu)on flow of the program The one- way and two- way if statements are, more specifically, condi)onal structures Itera)on structures are execu)on control structures that enable the repe))ve execu)on of a statement or a block of statements The for loop statement is an itera)on structure that executes a block of code for every item of a sequence

14 for loop Executes a block of code for every item of a sequence If sequence is a string, items are its characters (single- character strings) name = char = char = char = char = char = 'A p p l e' 'A' 'p' 'p' 'l' 'e' name = 'Apple' for char in name: print(char) A p p l e

15 for loop Executes a code block for every item of a sequence Sequence can be a string, a list, Block of code must be indented for <variable> in <sequence>: <indented code block > <non-indented code block> word = 'stop' for word in ['stop', 'desktop', 'post', 'top']: if 'top' in word: print(word) print('done.') word = word = word = 'desktop' 'post' 'top' stop desktop top Done.

Introduc)on to Compu)ng Using Python Built- in func'on range() Func)on range() is used to iterate over a sequence of numbers in a specified range To iterate over the n numbers 0, 1, 2,, n- 1 for i in

16 Introduc)on to Compu)ng Using Python Built- in func'on range() Func)on range() is used to iterate over a sequence of numbers in a specified range To iterate over the n numbers 0, 1, 2,, n- 1 for i in range(n): To iterate over the n numbers i, i+1, i+2,, n- 1 for i in range(i, n): To iterate over the n numbers i, i+c, i+2c, i+3c,, n- 1 for i in range(i, n): for i in range(4): range(0): range(1): range(0, range(2, 3): 2): 6): 16, 4): 10): print(i)

17 Exercise Write for loops that will print the following sequences: a) 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 b) 1, 2, 3, 4, 5, 6, 7, 8, 9 c) 0, 2, 4, 6, 8 d) 1, 3, 5, 7, 9 e) 20, 30, 40, 50, 60

Defining new func'ons A few built- in func)ons we have seen: abs(),

def: func)on defini)on keyword f: name of func)on def f(x): res = x**2

4) 4 lst = [2,3,4,5] len(lst) 4 sum(lst) 14 print() def f(x): res =

18 Defining new func'ons A few built- in func)ons we have seen: abs(), max(), len(), sum(), print() New func)ons can be defined using def def: func)on defini)on keyword f: name of func)on def f(x): res = x** return res x: variable name for input argument abs(-9) 9 max(2, 4) 4 lst = [2,3,4,5] len(lst) 4 sum(lst) 14 print() def f(x): res = 2*x + 10 return x** f(1) 11 f(3) 19 f(0) 10 return: specifies func)on output

print() versus return def f(x): res = x**2 + 10 return res def f(x): res = x**2 + 10 print(res) f(2)

<module> 2*f(2) TypeError: unsupported operand type(s) for *: 'int' and 'NoneType' Func)on returns

19 print() versus return def f(x): res = x** return res def f(x): res = x** print(res) f(2) 14 2*f(2) 28 f(2) 14 2*f(2) 14 Traceback (most recent call last): File "<pyshell#56>", line 1, in <module> 2*f(2) TypeError: unsupported operand type(s) for *: 'int' and 'NoneType' Func)on returns value of res which can then be used in an expression Func)on prints value of res but does not return anything

two numbers as input (side lengths a and b of above right triangle ) Returns the length of

20 Defining new func'ons The general format of a func)on defini)on is a c def <function name> (<0 or more variables>): <indented function body> b Let s develop func)on hyp() that: Takes two numbers as input (side lengths a and b of above right triangle ) Returns the length of the hypotenuse c hyp(3,4) 5.0 import math def hyp(a, b): res = math.sqrt(a**2 + b**2) return res

21 Exercise Write func)on hello() that: takes a name (i.e., a string) as input prints a personalized welcome message Note that the func)on does not return anything hello('julie') Welcome, Julie, to the world of Python. def hello(name): line = 'Welcome, ' + name + ', to the world of Python. Python.' print(line)

22 Exercise Write func)on rng() that: takes a list of numbers as input returns the range of the numbers in the list The range is the difference between the largest and smallest number in the list rng([4, 0, 1, -2]) 6 def rng(lst): res = max(lst) - min(lst) return res

res # compute result # and return it def f(x): 'returns x**2 + 10' res = x**2 + 10 return res Docstring def f(x): 'returns x**2

23 Comments and docstrings Python programs should be documented So the developer who writes/maintains the code understands it So the user knows what the program does Comments help(f) Help on function f in module main : f(x) def f(x): res = x** return res # compute result # and return it def f(x): 'returns x**2 + 10' res = x** return res Docstring def f(x): 'returns x**2 + 10' res = x** # compute result return res # and return it help(f) Help on function f in module main : f(x) returns x**2 + 10

Assignment statement: a second look a b c d A variable does not exist

3 'three' <variable> = <expression> [1, 2, 3] a 1.

24 Assignment statement: a second look a b c d A variable does not exist before it is assigned 'three' <variable> = <expression> [1, 2, 3] a 1. <expression> is evaluated and its value put into an object of appropriate type 2. The object is assigned name <variable> a Traceback (most recent call last): File "<pyshell#66>", line 1, in <module> a NameError: name 'a' is not defined a = 3 b = c = 'three' d = [1, 2] + [3]

Mutable and immutable types a b c d 3 6 3.

25 Mutable and immutable types a b c d 'three' [1, 2, 7, 3] The object (3) referred to by variable a does not change; instead, a refers to a new object (6) Integers are immutable a 3 a = 6 a 6 d [1, 2, 3] d[1] = 7 d [1, 7, 3] The object ([1, 2, 3]) referred to by d changes Lists are mutable

Assignment and mutability a b c d 9 3 [1, 7, 3] 9] 'three' 3.

3 b = a b 6 a = 9 b 6 c = d c [1, 7, 3] d[2] = 9 c [1, 7, 9] a The now list refers that c to refers a new to

26 Assignment and mutability a b c d 9 3 [1, 7, 3] 9] 'three' a c and b d refer to the same integer list object object a 6 b 3.3 b = a b 6 a = 9 b 6 c = d c [1, 7, 3] d[2] = 9 c [1, 7, 9] a The now list refers that c to refers a new to object changes; (9); d b refers s)ll refers to the to same the old list object, (6) so it changes too Because integers lists are are mutable, immutable, a change a change to d to affects a does c not affect the value of b

27 Swapping values Have: a b tmp Want: a b a 3 b 6 tmp a, b = b = a= b b, a b = a a = tmp

28 Immutable parameter passing a x shell Func)on call g(a) 3 5 def g(x): x = 5 a = 3 g(a) Func)on Variable x g() inside did g() not, refers and cannot, to the modify object a the refers value to of a in the interac)ve shell. As if we executed x = a This is because a refers to an immutable object.

modify refers the to value the object of lst lst in the refers interac)ve to

29 Mutable parameter passing lst l shell Func)on call h(lst) [1,2,3] [5,2,3] def h(l): l[0] = 5 lst = [1,2,3] h(lst) Func)on Variable l h() inside did h() modify refers the to value the object of lst lst in the refers interac)ve to shell. This As if is we because executed lst l and = lst l refer to an mutable object.

anything mylst = ['one', 'two', 'three'] swapfs(mylst) mylst ['two', 'one', 'three'] mylst =

30 Exercise Write func)on swapfs() that: takes a list as input swaps the first and second element of the list, but only if the list has at least two elements The func)on does not return anything mylst = ['one', 'two', 'three'] swapfs(mylst) mylst ['two', 'one', 'three'] mylst = ['one'] swapfs(mylst) mylst ['one'] def swapfs(lst): if len(lst) > 1: lst[0], lst[1] = lst[1], lst[0]

Execu&on Control Structures

Execu&on Control Structures Condi)onal Structures Itera)on Pa9erns, Part I Two- Dimensional Lists while Loop Itera)on Pa9erns, Part II One- way if statement if :